With the occurrence of telecommunication networks and in particular due to the huge spreading of multimedia data-capable personal computers and, most recently, of so-called solid state players, a need has arisen to market digital multimedia data, such as digital audio data and/or digital video data, commercially. Telecommunication networks for example can be analog telephone lines, digital telephone lines, such as ISDN, or the Internet. Among the commercial providers of multimedia products there is a need to sell or lend multimedia data, wherein it should be possible for a customer to be able to select a certain product individually at any time from a certain catalogue, this product then of course being only allowed to be used by the customer who has paid for it.
Unlike well-known encrypted television programs, such as the television channel Premiere, in which the emitted data is encrypted in the same way for all users who have acquired a suitable decryption device by paying a certain charge, the present invention is to provide methods and devices enabling an individual, customer-selective and safe encryption and decryption of multimedia data. Unlike the television channels mentioned above which give a fixed program all of which the user has to decide for, the methods and devices of the present invention enable a maximum freedom of selection for the user, which means that the user has only to pay for those products he or she actually wants to use.
DE 196 25 635 C1 describes methods and devices for encrypting and decrypting multimedia data, the multimedia data being present in the form of an encrypted multimedia file comprising a destination data block and a payload data block. Parts of the destination data block and at least some parts of the payload data block are encrypted by means of different keys, especially symmetrical encryption methods being used.
Symmetrical encryption methods on the one hand have the advantage that they can work relatively quickly, on the other hand the user who wants to decrypt the file needs the same key as the provider, such as the German company Deutsche Telekom, who has encrypted the multimedia data to sell it to the customer. Thus, both the provider and the user, that is the customer, on the one hand have a table with many possible symmetrical encryption algorithms, such as DES or Blowfish, and on the other hand a table for possible keys in such a way that the provider generates an entry into the destination data block of the multimedia data, the entry being used by the user to access his key table to select the correct key for the encryption.
Due to the rapidly increasing spreading of the MP3 standard so-called solid-state-players have emerged on the market, these players being used for decrypting and replaying multimedia data. These instruments are intended to be good value and thus can only comprise a limited amount of memory space and computing power. Unlike personal computers in which the resources present exceed the resources necessary for decrypting multimedia by far, solid-state-players or stereo systems are car hi-fi instruments must be good value in order to be competitive on the hard fought for market. In addition it is necessary to relieve these instruments, concerning the computing power and memory space, as far as possible when decrypting and replaying the decrypted multimedia data. On the other hand there is still the demand that the encryption techniques used be adequately safe to be trustworthy for the customer and to prevent an abuse even of encrypted multimedia data. In addition copyright violations are to be fought effectively, especially when multimedia data is replayed without an authorization by the author or the commercialisation company respectively or when it is changed without having an authorization to do so.